1. Field of the Invention
This invention relates to a semiconductor laser as a main component of optical communication systems.
2. Description of the Prior Art
Recently, there is a rapidly growing tendency toward spreading the application field of optical communications from the key transmitting system to the optical communication systems including the subscriber system, local area network (LAN) system and data-link system accompanied with the progression of the optical communication technology. In order to respond to such highly advanced optical communication systems, the use of high performance optical devices unavoidably becomes a critical factor for success.
Such a semiconductor laser that has an In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y /InP double-hetero structure has been realized previously as a light source for the low-loss optical fiber communication purpose. However, with a p-type InP/In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y /n-type InP double-hetero structure semiconductor laser with a wave length band of 1 .mu.m, there arise problems that the luminous efficiency is low, the optical output is saturated with a low output and the operation at an elevated temperature is difficult because the characteristic temperature TO of oscillating threshold current is low compared with a GaAlAs/GaAs system semiconductor laser with a wavelength band of 0.8 .mu.m as pointed out by Yonezu (refer to "Optical Communication Device Technology--Light Emitting and Receiving Devices", pp. 225-243, 1984, published by Kogaku-Tosho Corp.).
One reason therefor is that the electrons injected into an active layer (light emitting layer) are re-combined with the holes without contributing to the light emission due to the leakage process of a carrier in a cladding layer, the Auger process and the like. With the InxGa.sub.1-x As.sub.y P.sub.1-y /InP system semiconductor laser, the value of .DELTA.Ec/.DELTA.Ev, which is a ratio of the amount of band discontinuity .DELTA.Ec of conduction band and the amount of discontinuity .DELTA.Ev of valence band, namely, so-called band offset ratio, is so small as 0.22/0.38 that the electrons injected into the active layer of InGaAs or InGaAsP easily pass through the hetero barrier and leak in the p-type InP cladding layer.
Particularly, in a semiconductor laser having an InGaAsP active layer, the electrons having a high kinetic energy are easily generated in the active layer through the Auger recombination process, so that the leakage of the electrons to the p-type InP cladding layer is accelerated. In addition, when operated at an elevated temperature, the electrons with a high energy will be generated into the InGaAsP active layer, so that the leakage of the electrons to the p-type InP cladding layer will be further accelerated.
Thus, an object of this invention is to provide a semiconductor laser having an InGaAsP layer or InGaAs layer as an active layer which has a high luminous efficiency and output performance as well as the capability of being operated at an elevated temperature.